1. Field of the Invention
This invention relates to a fuel injection control system for an internal combustion engine, for controlling the amount of fuel injected from a fuel injection valve by a fuel injection period over which the valve is opened for fuel injection.
2. Description of the Prior Art
Conventionally, a fuel injection control system of the above-mentioned kind has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 7-166922 which discloses an internal combustion engine of an in-cylinder injection type which injects fuel directly into combustion chambers. In the proposed fuel injection control system, fuel behavior parameters indicative of behaviors of injected fuel are calculated and a fuel injection amount Fi is determined by using these fuel behavior parameters, in the following manner: A combustion-chamber fuel deposition ratio xcex1c is retrieved from a map according to the engine temperature TE and the engine rotational speed NE. Further, a cylinder fuel deposition ratio xcex1d is determined according to the engine temperature TE and the engine coolant temperature TW, and a gaseous fuel ratio xcex2c is calculated from the combustion-chamber fuel deposition ratio xcex1c and the cylinder fuel deposition ratio xcex1d.
Then, a fuel evaporation ratio xcex3c is retrieved from a map according to the engine temperature TE and the engine rotational speed NE, and an exhaust carry-off ratio xcex3e is calculated from the fuel evaporation ratio xcex3c. Further, a gaseous fuel residual ratio xcex2k is retrieved from a map according to the engine temperature TE and the engine rotational speed NE. Then, the present value of a combustion-chamber gaseous fuel amount Fc is calculated from the engine rotational speed NE and an intake air amount Q. The fuel injection amount Fi is calculated from the present value of the combustion-chamber gaseous fuel amount Fc, the aforementioned fuel behavior parameters, and the immediately preceding value of a combustion-chamber deposited-fuel amount Mc. Further, a fuel injection period Tout is calculated from the fuel injection amount Fi.
Another internal combustion engine of the in-cylinder injection type is also known which injects fuel during a compression stroke and performs stratified combustion while controlling the air fuel ratio of a mixture to a far leaner value than a stoichiometric air-fuel ratio, under a very low load operating condition, such as idling. In this kind of internal combustion engine, fuel is supplied to a fuel injection valve in a state pressurized by a fuel pump and fuel injection is carried out during the compression stroke, so that the fuel pressure is set to a much higher level than when fuel injection is carried out during the intake stroke.
As described above, in the above conventional fuel injection control system, the fuel injection period Tout is calculated by using the engine temperature TE, the engine rotational speed NE, the engine coolant temperature TW, and the intake air amount Q as parameters. However, the amount of fuel actually injected into each cylinder varies with the fuel pressure, even if the fuel injection period Tout is not changed. For example, as the fuel pressure increases, the amount of actually injected fuel increases. In spite of this phenomenon, the conventional fuel injection control system only uses the above-mentioned parameters in calculation of the fuel injection period Tout, so that the fuel pressure cannot be reflected in the calculation, and hence the fuel injection period Tout cannot be properly calculated. Particularly in the case of the above in-cylinder injection type which performs stratified combustion, the fuel pressure changes largely e.g. immediately after fuel injection since the pressure is inherently set to a very high level, so that the actual fuel injection amount tends to largely deviate from the optimum amount. As a result, e.g. in the execution of feedback control of the air-fuel ratio, the fuel injection amount cannot be properly controlled with respect to a target air-fuel ratio, which degrades convergence of the air-fuel ratio control.
It is an object of the invention to provide a fuel injection control system and method for an internal combustion engine as well as an engine control unit, which are capable of accurately determining a fuel injection period such that fuel pressure and fuel deposition are appropriately reflected therein, thereby performing accurate and optimum control of the actual fuel injection amount.
To attain the above object, according to a first aspect of the present invention, there is provided a fuel injection control system for an internal combustion engine, for controlling the amount of fuel to be injected from a fuel injection valve, by a fuel injection period during which the fuel injection valve is opened for fuel injection,
the fuel injection control system comprising:
operating condition-detecting means for detecting operating conditions of the engine;
demanded fuel amount-determining means for determining an amount of fuel demanded by the engine, according to the detected operating conditions of the engine;
deposited fuel amount-determining means for determining, out of an amount of fuel injected from the fuel injection valve, an amount of fuel deposited on walls downstream of the fuel injection valve, according to the detected operating conditions;
net fuel amount-determining means for determining a net amount of fuel to be injected from the fuel injection valve, based on the determined demanded amount of fuel and the determined deposited amount of fuel;
fuel pressure-determining means for determining pressure of fuel to be injected from the fuel injection valve; and
fuel injection period-determining means for determining the fuel injection period by correcting the determined net amount of fuel according to the determined pressure of fuel.
According to this fuel injection control system, the amount of fuel demanded by the engine, and the amount of fuel deposited on walls downstream of the fuel injection valve out of the amount of fuel injected from the fuel injection valve are determined according to the operating conditions of the engine. Further, the net amount of fuel to be injected from the fuel injection valve is determined based on the determined demanded amount of fuel and the determined deposited amount of fuel. Then, by correcting the determined net amount of fuel according to the pressure of fuel, the fuel injection period is determined. The deposited fuel is burned within the combustion chamber after being once deposited on the walls downstream of the fuel injection valve, e.g. those of an intake port and a combustion chamber. Therefore, a proportion of an amount of burned fuel of the deposited fuel to the whole amount of the deposited fuel is inherently indifferent to the fuel pressure. Therefore, by determining the net amount of fuel to be injected based on the demanded amount of fuel and the deposited amount of fuel, as described above, without carrying out the correction dependent on the fuel pressure, it is possible to properly determine the net amount of fuel by causing the operating conditions of the engine to be reflected therein, while taking the deposited fuel amount into account and at the same time excluding the influence of fuel pressure. Further, the net amount of fuel thus determined is corrected according to the pressure of fuel to determine the fuel injection period. This makes it possible to properly compensate for fuel pressure-dependent variation in the amount of fuel actually injected from the fuel injection valve. As a result, according to the first aspect of the invention, it is possible to accurately determine the fuel injection period such that fuel pressure and fuel deposition are appropriately reflected therein, thereby perform accurate and optimum control of the actual fuel injection amount.
The engine includes a cylinder defining a combustion chamber therein, and it is preferred that the fuel injection valve is arranged such that fuel is injected directly into the combustion chamber of the cylinder.
As described above, an internal combustion engine of the in-cylinder injection type which injects fuel directly into the combustion chamber has characteristics that the fuel pressure is set to a very high level and undergoes a large variation. Therefore, according to this preferred embodiment, by causing the largely varying fuel pressure to be properly reflected in the determination of the fuel injection period, the advantageous effects described above can be effectively obtained for this type of engine.
More preferably, the fuel pressure-determining means includes supply fuel pressure-detecting means for detecting pressure of the fuel supplied to the fuel injection valve, and internal cylinder pressure-determining means for determining pressure within the combustion chamber, and determines the pressure of fuel as a difference between the pressure of fuel detected by the fuel pressure-detecting means and the pressure within the combustion chamber determined by the internal cylinder pressure-determining means.
Further preferably, the internal cylinder pressure-determining means includes crank angle-detecting means for detecting a crank angle of the engine, and determines the pressure within the combustion chamber according to the detected crank angle.
To attain the above object, according to a second aspect of the invention, there is provided a fuel injection control method for an internal combustion engine, for controlling the amount of fuel to be injected from a fuel injection valve, by a fuel injection period during which the fuel injection valve is opened for fuel injection,
the fuel injection control method comprising the steps of:
detecting operating conditions of the engine;
determining an amount of fuel demanded by the engine, according to the detected operating conditions of the engine;
determining, out of an amount of fuel injected from the fuel injection valve, an amount of fuel deposited on walls downstream of the fuel injection valve, according to the detected operating conditions;
determining a net amount of fuel to be injected from the fuel injection valve, based on the determined demanded amount of fuel and the determined deposited amount of fuel;
determining pressure of fuel to be injected from the fuel injection valve; and
determining the fuel injection period by correcting the determined net amount of fuel according to the determined pressure of fuel.
According to the second aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
The engine includes a cylinder defining a combustion chamber therein, and it is preferred that the fuel injection valve is arranged such that fuel is injected directly into the combustion chamber of the cylinder.
According to this preferred embodiments, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
More preferably, the step of determining the pressure of fuel includes the steps of detecting pressure of the fuel supplied to the fuel injection valve, determining pressure within the combustion chamber, and determining the pressure of fuel as a difference between the detected pressure of fuel and the determined pressure within the combustion chamber.
Further preferably, the step of determining the pressure within the combustion chamber includes detecting a crank angle of the engine, and determining the pressure within the combustion chamber according to the detected crank angle.
To attain the above object, according to a third aspect of the invention, there is provided an engine control unit including a control program for causing a computer to carry out fuel injection control for controlling an amount of fuel to be injected into an internal combustion engine from a fuel injection valve, by a fuel injection period during which the fuel injection valve is opened for fuel injection,
wherein the control program causes the computer to detect operating conditions of the engine, determine an amount of fuel demanded by the engine, according to the detected operating conditions of the engine, determine, out of an amount of fuel injected from the fuel injection valve, an amount of fuel deposited on walls downstream of the fuel injection valve, according to the detected operating conditions, determine a net amount of fuel to be injected from the fuel injection valve, based on the determined demanded amount of fuel and the determined deposited amount of fuel, determine pressure of fuel to be injected from the fuel injection valve, and determine the fuel injection period by correcting the determined net amount of fuel according to the determined pressure of fuel.
According to the third aspect of the invention, the same advantageous effects as provided by the first aspect of the invention can be obtained.
The engine includes a cylinder defining a combustion chamber therein, and it is preferred that the fuel injection valve is arranged such that fuel is injected directly into the combustion chamber of the cylinder.
According to this preferred embodiment, the same advantageous effects as provided by the corresponding preferred embodiment of the first aspect of the invention can be obtained.
More preferably, when the control program causes the computer to determine the pressure of fuel, the control program causes the computer to detect pressure of the fuel supplied to the fuel injection valve, determine pressure within the combustion chamber, and determine the pressure of fuel as a difference between the detected pressure of fuel and the determined pressure within the combustion chamber.
Further preferably, when the control program causes the computer to determine the pressure within the combustion chamber, the control program causes the computer to detect a crank angle of the engine, and determine the pressure within the combustion chamber according to the detected crank angle.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.